This is a breaking news story. Check back for updates.
Update 4:15 p.m. Wednesday EDT: Officials at three Boston area hospitals told the Associated Press that they expect all of their patients being treated for injuries sustained in the bombings to survive.
"We have a lot of lower extremity injuries, so I think the damage was low to the ground and wasn't up," said Dr. Peter Burke, chief of trauma surgery at Boston Medical Center, where about 25 victims were treated. "The patients who do have head injuries were blown into things or were hit by fragments that went up."
At Massachussets General Hospital, the lion's share of the 31 people treated for injuries have been released, a spokeswoman told the AP. Of the 35 patients treated at Brigham and Women's Hospital, none have life-threatening injuries. Half of the 14 patients taken to Tufts Medical Center have also been released. You can get a full roundup of Boston area hospitals here.
Three people were killed in the bombings: Martin Richard, 8, of Boston; Krystle Campbell, 29, of Arlington, Mass.; and Boston University graduate student Lingzi Lu of China.
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Update 1:41 p.m. Wednesday EDT: An arrest in the Boston Marathon bombings has either already taken place or is "imminent," Boston television station WCVB reports:
â€” WCVB-TV Boston (@WCVB) April 17, 2013
Update 1:21 p.m. Wednesday EDT: Surveillance video from a department store near the blasts helped the FBI identify the possible suspect in the Boston Marathon bombings, according to CNN. A Boston television station also provided video that was helpful to the FBI.
Update 1:13 p.m. Wednesday EDT: Authorities believe they have identified a suspect in the bombings, a source told CNN. Authorities have also detected an image of a suspect carrying and possibly dropping a black bag at the site of the second Boston Marathon explosion, the Boston Globe reports. It's unclear whether it's the same suspect.
Update 12:46 p.m. Wednesday EDT: White House spokesman Jay Carney told reporters that First Lady Michelle Obama would be accompanying the president to Boston for a memorial service Thursday dedicated to the victims and survivors of the Boston explosions. He also gave the following glimpse into what the president's message will be to the nation in Boston:
"It will be one of resolve. It will be one of the commonality that we feel as Americans with the people of Boston and those that were visiting Boston for the marathon ... and then demonstrated bravery in its immediate aftermath." Carney said the response to the bombings "reminds us and reminds the world who we are as a people."
Meanwhile, the FBI said there is no indication that the suspcious letters and packages sent to Obama and two U.S. senators are connected to the explosions.
The investigation into these letters remains ongoing, and more letters may still be received. There is no indication of a connection to the attack in Boston," the agency said in a statement.
Update 11:18 a.m. Wednesday EDT: Investigators have discovered the lid of one of the pressure cookers that housed bombs used in the explosions, an official told the Associated Press. The lid was recovered from the roof of a nearby building. The FBI said Tuesday that the bombs were placed in pressure cookers that were either contained in backpacks or nylon bags.
Update 10:31 a.m. Wednesday EDT: Witnesses to the explosion who captured even partial video recordings of the blasts are being urged to send the videos to the FBI. The agency said these clips can be beneficial to the investigation, even if the owners of the recordings do not think so.
Update 8:25 a.m. Wednesday EDT: The FBI is appealing to individuals who may have photos or video of the bombings. "Someone knows who did this," FBI Agent in charge Richard DesLauriers said, Reuters reported. He added that community cooperation will play an important role in the investigation.
Update 1:45 a.m. Wednesday EDT: A regional Chinese newspaper has identified the third person killed in the Boston Marathon bombings as Lu Lingzi, a graduate student at Boston University originally from the northeastern city of Shenyang.
The Shenyang Evening News reported the name Wednesday on its official Twitter-like microblog. An editor at the paper said Lu's father confirmed her death when reporters visited the family home, the Associated Press reports.
The Chinese Foreign Ministry and Consulate General in New York did not release the victim's name at the request of the family. But Tuesday, Boston media quoted a Chinese consular official as saying Chinese national Lu Lingzi was missing after the bombings.
Update 11 p.m.: Initial evidence on the Boston bombings suggests homegrown terrorism, Sen. Saxby Chambliss of Georgia, the top Republican on the Senate Intelligence Committee, said after the panel was briefed by Director of National Intelligence James Clapper, Bloomberg reports.
“There are a lot of things that are surrounding this that would give an indication it may have been a domestic terrorist,” Chambliss said. Still, he said, officials can’t say for certain.
Rep. Dutch Ruppersberger of Maryland, the top Democrat on the House intelligence panel, said spy agencies suspect the Boston attacks were the acts of a “lone wolf.”
The lack of chatter or any information in advance of the attacks points to the possible acts of an individual or someone who worked “very quietly” with a few people, he said, though he cautioned that’s still a crucial factor the investigators have to determine.
Update 9 p.m.: The Chinese Consulate in New York has confirmed that the third person killed in Monday’s Boston Marathon bombing was a Chinese woman. According to the Boston Globe, she was a Boston University graduate student, though the university has not released her name, honoring the family's request.
A second Chinese national and BU student was also injured in the bombing, but remains in stable condition. She was identified as Zhou Danling of central China. “She cannot talk now but can communicate with pen and paper,” the consulate told the Boston Globe.
The agency announced Tuesday that all residents of Suffolk County, Mass., including the city of Boston, can take the additional time to file. People affected by bombing who live elsewhere can claim the extension by calling the IRS at 1-866-562-5227, starting April 23.
“Our hearts go out to the people affected by this tragic event,” said acting IRS Commissioner Steven T. Miller. “We want victims and others affected by this terrible tragedy to have the time they need to finish their individual tax returns.”
Update 6:55 p.m.: President Obama will visit Boston on Thursday for a memorial service dedicated to the victims and survivors of the bombings, says White House Press Secretary Jay Carney.
Carney said the event would be "an interfaith service dedicated to those who were gravely wounded or killed in Monday's bombing," USA Today reported.
Update 6:03 p.m.: A Boston University graduate student has been identified as the third person to die in the Boston Marathon bombings, the school said Tuesday afternoon.
The university is not releasing the student's name pending permission from their family. The student went to the marathon with two friends, also students at the school, to watch the race by the finish line. One friend was injured in the blasts and is in stable condition at Boston University Medical Center; the other was unharmed.
Also killed in the bombings were 29-year-old Krystle Campbell of Medford, Mass., and 8-year-old Martin Richard of Boston.
Update 5:34 p.m.: On the human interest side of the tragedy, Massachusetts Gov. Deval Patrick wrapped up the news conference by making a plea for a veteran of the war in Afghanistan only known as "Tyler" to contact "Victoria," a woman injured in the blasts who said the soldier comforted her as she was being tended to shortly after the explosion.
"She was scared, she was carried we think by a firefighter, to the medical tent and [was] really -- she described it -- hysterical," the governor said of Victoria, a Northeastern University student being treated for serious shrapnel wounds in her legs.
"Victoria very much wants to thank Tyler personally ... We would love to hear from Tyler so we can connect him to Victoria," Patrick said, urging the veteran to call 617-725-4000 --- the number of the governor's office.
Update 5:13 p.m.: Rick DesLauriers, FBI special agent in charge for Boston, said bomb examiners are reconstructing the devices, which contained "pieces of black nylon" and may have been placed in a dark colored backpack. He said the bombs contained BBs, nails and were "possibly contained in a pressure cooker device."
DesLauriers told a news conference that authorities could not specify at this time specific components used in the making of the bombs. He added that the FBI received more than 2,000 tips on the bombings since noon Tuesday.
No suspects have been identified in the attacks, he said.
"At this time, there are no claims of responsibility," DesLauriers said. "The range of suspects remain wide open."
The FBI is particularly interested in tips from people who encountered someone who spoke about the marathon or mentioned the date of April 15 and indicated that they would target the marathon.
"Someone knows who did this," DesLauriers said.
He said people should look out for those who had "any expressed desire" or a "specific interest in creating explosives." Anyone who heard noises in remote areas that might have been locations to test out bombs should also come forward, as well as anyone who noticed someone with a dark colored bag.
Update 4:54 p.m.: The number of victims in the explosion has risen to 183, with 23 people in critical condition, according to CNN. Nine children are among those injured, and three people have died in the blast, including 8-year-old Martin Richard.
Update 3:29 p.m.: The first bomb that exploded was located in the viewing stand where Massachusetts Gov. Deval Patrick was watching the marathon earlier in the day, the Boston Globe reports:
First bomb located near viewing stand where Gov Patrick sat earlier in day, Globe's @shelleymurph reports.
â€” The Boston Globe (@BostonGlobe) April 16, 2013
Update 3:05 p.m.: U.S. Homeland Security Director Janet Napolitano says so far there's no evidence that the Boston bombings are connected to a larger terrorism plot, according to Reuters:
U.S. Homeland Security Secretary Napolitano says no indication Boston bombing part of broader plot #breaking
â€” Reuters US News (@ReutersUS) April 16, 2013
Update 2:57 p.m.: A circuit board believed to have been used to trigger the Boston explosions has been recovered, the Boston Globe tweets:
BREAKING NEWS: Source: Investigators found circuit board believed used to trigger marathon bombs.via @shelleymurph
â€” The Boston Globe (@BostonGlobe) April 16, 2013
Update 2:34: Campbell, the manager of a restaurant in Arlington, Mass., went to the marathon to take pictures of her friend's boyfriend crossing the finish line, her father, William Campbell, tells the Associated Press. The friend was seriously injured in the explosion.
Update 2:08 p.m.: The Boston Globe has identified Krystle Campbell, 29, of Medford, Mass., as one of three people who died in the explosions. Previously, authorities identified 8-year-old Martin Richard of Boston as one of the three people killed in the bombings.
BREAKING NEWS: Second victim identified in Boston Marathon bombings: Krystle Campbell of Arlington.
â€” The Boston Globe (@BostonGlobe) April 16, 2013
Update 1:01 p.m.: The Washington Post reports two Saudis were victims of the Boston attacks -- the previously mentioned Saudi woman and the man who was questioned. The man, who sustained severe burn injuries in the bombings, voluntarily allowed authorities to search his apartment in connection with the attacks and is considered to be a witness, not a suspect, according to the Post. No Saudis are being treated as suspects, an unnamed Saudi official told the paper.
“We’re not aware of any Saudi suspect or Saudi person of interest,” they said.
Update 12:43 p.m.: A Saudi official says only one native of Saudi Arabia has been confirmed as a casualty in the Boston explosions and identified the victim as a woman. The Boston Globe previously reported that a man was injured in the explosion and later question by the FBI in connection with the attacks. Other reports indicated there were two Saudi natives injured in the attacks.
According to Arab News, the Saudi woman who was the lone confirmed casualty was identified as Noura Al-Ajaji. Mohammad Al-Eisa, head of the Saudi Cultural Attaché, said Al-Ajaji was "slightly injured in the attack.
"She was at a hospital and had assured her husband by phone that she would be discharged soon," he said.
Update 12:19 p.m.: The bombs used in the explosions were housed in 6-liter pressure cookers and wrapped in black duffel bags, a person briefed on the investigation told the Associated Press. The explosives contained nails, ball bearings and metal scraps, the source said, which backs up comments from Boston doctors who said they treated patients who had those items removed from them during surgery.
Update 11:34 a.m.: President Barack Obama told an 11:30 a.m. news conference that it is still unknown who carried out the attack or whether it was committed by a terrorist group or a "malevolent individual." But the president did describe the bombings as terrorist attacks.
"This was a heinous and cowardly act, and given what we know of what took place, the FBI is investigating it as an act of terrorism," the president said. "We will find whoever harmed our citizens and we will bring them to justice," he added. "We also know this: The American people will refuse to be terrorized. ... In the coming days we will pursue every effort to get to the bottom of what happened and we will remain vigilant."
Update 11:26 a.m.: LaGuardia passengers are being allowed back into the central terminal, according to NBC New York:
â€” NBC New York (@NBCNewYork) April 16, 2013
Update 11:00 a.m.: The central terminal at LaGuardia Airport in New York City has been evacuated due to a suspicious package, CBS New York reports. Meanwhile, a U.S. Airways flight has been grounded at Logan Airport in Boston for security reasons, Bloomberg News tweets:
BREAKING: US Airways Flight 1716 stopped on ground at Boston's Logan Airport for security
â€” Bloomberg News (@BloombergNews) April 16, 2013
Update 10:10 a.m.: Davis says there are 176 casualties, including three deaths and 17 in critical condition. He said witnesses with photos can be helpful to police if they tell authorities what time the pictures were taken "so we don't have to go through the time signatures" and save time on the investigation.
Update 9:58 a.m.: Boston Police Commissioner Ed Davis said the crime scene has been reduced from 15 to 12 blocks in Copley Square. He urged witnesses to provide any videos or photos that were taken during the explosions, saying they "could be helpful to this investigation.
"Our focus is on processing that evidence right now," he said.
Col. Timothy Alben, superintendent of the Massachusetts State Police, said there could be "hundreds, if not thousands, of photographs" taken of the scene.
"I would encourage you to bring forward anything," he said. "You might not think it's significant, but it might have some value to this investigation."
Suffolk County District Attorney Daniel Conley said the attacks were cowardly.
"What occurred yesterday in Boston was an act of cowardice," he said. "An act of cowardice, and of this severity, cannot be justified or explained -- it can only be answered."
Update 9:50 a.m.: Rick DesLauriers, FBI special agent in charge for Boston, said there are no known additional threats against the city. He said the agency is in process of interviewing witnesses.
"We have received voluminous tips in the 18 hours since the incidents," DesLauriers said.
Gene Marquez, acting special agent in charge for the ATF's Boston Field Division, said the agency is bringing explosive specialists to the scene.
Update 9:44 a.m.: Massachusetts Gov. Deval Patrick said information that two unexploded devices were found is inaccurate. Boston Mayor Thomas Menino said a resource center was being set up near the Park Plaza Hotel.
"This is a bad day for Boston, but I think if we pull through together we'll get through it," he said.
U.S. Sen. Elizabeth Warren, D-Mass., reiterated that sentiment.
"As the mayor said, Boston will survive," she said.
As of early Tuesday morning, authorities have no suspects in the Boston Marathon explosions that killed three people and injured at least 152 others.
Investigators searched a home Monday night in nearby Revere, Mass., in relation to the explosions that detonated in Copley Square near the finish line of the marathon, the Boston Globe reported. The paper reported that the FBI were looking for a "person of interest" believed to be living in the home but no arrests were made.
The search was conducted without a warrant, meaning investigators were given consent to enter the home, CNN reported.
A Saudi national was also questioned Monday night at a Boston hospital in connection with the bombings, but he was yet to be arrested, according to the Globe.
The Obama administration is treating the attacks as acts of terrorism, but so far no terrorist groups have claimed responsibility for the attacks that shook Boston and the rest of the country.
Investigators are still determining whether more than one person was responsible for the bombings. The FBI is the lead agency in the investigation.
"So far, investigators believe it was not the work of suicide bombers, but it is still too early to rule it out completely," a European security official told the Associated Press.
The bombings killed three people, including 8-year-old Martin Richard.